SOIL PREPARATION

penetrometer resistance only provides a relative measure of the soil resistance to root penetration. Typically, the soil resistance to a fixed penetrometer probe – rotating penetrometers have also been developed that have much lower resistance to penetration – will be two to eight times higher than to a root (Bengough et al ., 1997). In a thorough study, Cassel & Nelson (1979), however, concluded that penetrometer data can be validly used to assess soil strength, despite the fact that much variation may exist. Generally, root penetration decreases approximately linearly with increasing penetrometer resistance. Values of 2 000 to 2 500 kPa have been reported in many studies as the critical penetrometer resistance for various crops and penetrometer probes (Zimmerman & Kardos, 1961; Taylor & Burnett, 1964; Greacen et al ., 1969; Bar-Yosef & Lambert, 1981; Nel & Bennie, 1984: Bengough et al ., 2011). It will, however, be wrong to see 2 000-2 500 kPa as a threshold value, since some root penetration will still occur at higher penetrometer readings, but to an increasingly limited extent. Penetrometers can vary much in sophistication from home-made ones to mechanical instruments (Figure 3.4) to constant speed motor-driven instruments that automatically register soil strength continuously with depth (Figure 3.5). A home-made penetrometer can simply consist of a steel rod with a sharpened end and a handle to push it manually into the soil. Once the handler has gained some experience with the home-made penetrometer, it is possible to “feel” the resistance of compact layers or big clods while pushing the steel rod into wet soil.

FIGURE 3.4: Portable fixed penetrometer that records soil strength mechanically on a small card. In this particular instance the depth of soil preparation is assessed (Photos: P. Myburgh, ARC Infruitec-Nietvoorbij).

38 | DETECTION OF ROOT IMPEDIMENTS